Bisulfite, a common food preservative, has been previously reported to be mutagenic in a number of cellular systems and also in phage T4. Because phage T4 employs 5-hydroxymethylcytosine instead of ordinary cytosine in its DNA, however, and because 5HMC is chemically far more resistant to the mutagenically significant action of bisulfite normally observed with cytosine itself, the action of bisulfite upon T4 should be critically re-examined. The earlier T4 report has now been shown to be irreproducible. Furthermore, bilsulfite is not even mutagenic for bacteriophage T4 whose 5-HMC has been largely replaced by ordinary cytosine, even when the treated phages are grown on host cells unable to excise uracil residues from DNA.